Coated columbium or tantalum base metal



Dec. 27, 1966 s. FOLDES ETAL COATED GOLUMBIUM OR TANTALUM BASE METAL 2Sheets-Sheet 1 Filed Jan. 26, 1966 Cb Hlloy conT/Ne a) ALLOY fiFTEEPas/0N mEnTME/v r W M A w lnven tors S tephen FoLdes Moses A. Levinstein in M1.

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CQATED COLUMBIUM OR TANTALUM BASE METAL 2 Sheets-Sheet 2 Filed Jan. 26,1966 T w 7 a M a A m "Wu WW M MT T N m M m w w E PRESENCE OF EUTELT/CnFmQ .m H :%e m dw v T 0 T m w WM WA WhA Nw n mm TS T 09 Mb UnitedStates Patent 3,294,497 CUATED CQLUMMUM UR TANTALUM BASE METAL StephenFoldes, San Jose, Calif., and Moses A. Levinstein,

Cincinnati, Ohio, assignors to General Electric Company, a corporationof New York Filed .lan. 26, 1966, Ser. No. 523,220 7 Claims. (Cl.29--l94) This application is a continuation-in-part of applicantscopending application Ser. No. 232,896, now abandoned, filed October 24,1962.

This invention relates to an oxidation resistant article of a refractorymetal or metal alloy and more particularly to an aluminide coatingsystem on notch-sensitive refractory metals.

The oxidation resistance of a wide variety of metals and their alloyshas been improved through the use of the metal aluminum as a surfacecoating, usually alloyed with the base metal, at least at the interfacebetween the two. Such a coating may be referred to as an aluminumbasecoating or an aluminide coating because of the formation of an aluminideof the base metal, as for example a columbium aluminide. These coatingshave been applied in a variety of ways including the application ofpowdered material and by various molten spray and molten dip techniques,for example as shown in Patent 3,186,070, Gordon D. Oxx, patented June1, 1965 and assigned to the same assignee as the present invention.

It is well known that various alloys of aluminum can be used to provideoxidation resistant coatings on refractory metals, particularlycolumbium (also known as niobium) and tantalum and alloy systems basedon these metals. However, most of the coatings of this type currentlyknown are subject to the handicaps of being quite brittle, causingserious notch embrittlement of the base metal, or both. These eifectsare especially harmful to the high strength alloys of columbium, many ofwhich are inherently notch sensitive to some extent. A very brittlecoating also has the disadvantage of forming a network of fine crackswhich allow air to reach the substrate thereby causing contamination andresulting embrittlement of the base metal.

On the other hand, some of the compounds formed between refractorymetals and aluminum are highly oxidation resistant and thereforedesirable as coatings. For example, three intermetallic compounds can beprepared in the columbium-aluminum system, namely Cb Al, Cb Al, and CbAlOf the columbium aluminides CbAl is the most oxidation resistant as wellas the most brittle phase. Corresponding phase relations exist in thetantalum-aluminum system, and the coatings herein described are equallyuseful on columbium or columbium-base alloys and tantalum ortantalum-base alloys.

It is an object of this invention to provide a highly oxidationresistant article of columbium or tantalum or an alloy based on one ofthese metals having an oxidation resistant aluminum-base coating withminimum susceptibility to crack propagation through the coating or intothe base metal.

These and other objects and advantages will be recognized from thefollowing detailed description and examples and the accompanyingdrawing.

Briefly stated, in one form, the present invention provides an articleof a refractory metal or alloy having a first coating of silver bondedthereto of a thickness sutiicient to cover the entire surface and asecond coating of powder metal aluminum-base alloy bonded to said silvercoating. The composite body is heat treated to fuse the coatingstogether and to the base metal.

In another form, the present invention provides an article of columbiumor tantalum or an alloy of either,

3,204,497 Patented Dec. 27, 1966 ice on the surface of which has beenfused an aluminide coating in the form of either CbAl or TaAl whichaluminide coating includes silver in solid solution.

In another form, the present invention provides an oxidation resistantarticle of columbium or tantalum or an alloy of either having analuminide coating fused to the surface thereof, the coating beingpredominantly CbAl or TaAl with silver and another metal, preferablysilicon, in solid solution, the coating having a silver and silicon richportion adjacent the interface between the coating and the base metal.Upon exposure to air at elevated temperatures, an outer skin of A1 0forms, and an inner layer converts from CbAl or TaAl to Cb Al or Ta Al,respectively, with silver and silicon precipitating therefrom. There issome intermixing of these layers.

In each of these embodiments of the invention, an outer coating ofaluminum metal may be provided over an aluminum alloy coating.

In the drawings, FIG. 1 is a photomicrograph of a coating system of theinvention, and FIG. 2 is a photomicrograph of the same coating after anoxidation treatment. FIG. 3 is an electron photomicrograph at highmagnification illustrating the ductile eutectic formed in the region ofthe coating-base metal interface according to the invention. FIG. 4 isthe same as FIG. 3 but after a two-hour oxidation treatment at 2500 F.

Wherever Cb AI, CbAl Ta Al or TaAl are referred to in thisspecification, it should be understood that complex aluminides of theform M Al and MAl are meant to be included. M is representative of thebase metal which can be columbium, a columbium-base alloy, tantalum, ora tantalum-base alloy. For example, in a columbium-base alloy containing15 percent titanium, M would represent (Cb,Ti) and M Al would be (Cb,Ti)Al. In like manner, when other metals such as, for example, chromium,silicon or titanium are added to the aluminum alloy coating, the coatingwould contain the complex aluminides of columbium and some or all of theother metals present in either the coating or the base metal alloy.

Broadly stated the present invention preferably provides an article ofmanufacture comprising a body of a base metal selected from the groupconsisting of columbium, columbium-base alloys, tantalum, andtantalum-base alloys, and a protective coating integral with an exposedsurface of said body to protect it' against oxidation while notdestroying the ductility of the base metal by greatly increasedsensitivity to crack propagation, said coating being bonded together andto said base metal by diffusion and comprising: a continuous layer of acontinuous phase of a monometal trialuminide having a composition in theproportion of one atom at least predominantly of the selected one of thebase metals columb'um and tantalum to each three atoms of aluminum, saidlayer containing sufiicient amounts of silver in solution so that asexcess of the selected one of the base metals columbium and tantalumdiffuses into said layer upon exposure to elevated temperatures and anoxidizing atmosphere, a ductile eutectic containing silver, aluminum andthe selected one of the base metals columbium and tantalum forms at asubstantial portion of the grain boundaries of said monometaltri-aluminide and acts as a barrier to the propagation of cracks fromsaid layer into said base metal.

Upon continued exposure to elevated temperatures such as at about 2500F. for about two hours in an oxidizing atmosphere, a continuous layer ofA1 0 forms at the exposed surface of the monometal trialuminide, therebydepicting the aluminum content of the aluminide, and the columbium ortantalum of the base metal diffuses out into the coating, causing theformation of a di-metal aluminide having a composition in the proportionof two atoms at least predominantly of the selected one of the aluminum.

and the layer grows in thickness with time.

base metals columbium and tantalum to each atom of This di-metalaluminide forms into a continuous layer at the interface between thebase metal and the monometal trialuminide of the balance of the coating,Ultimately, in the case of columbium and columbium-base alloys, When theCbAl has entirely converted to Cb Al, the excellent oxidation resistancecharacteristics of Cb Al are no longer available. Likewise, over a longperiod of time, as oxygen diffuses into and reacts with the Cb Al, it isgradually consumed by conversion into A1 Analogous effects take place incoated tantalum and tantalumbase alloys.

When the base metal is columbium or a columbiumbase alloy, the monometaltrialuminide and di-metal aluminide will be predominantly CbAl and Cb Alrespectively. Other elements present in the base metal alloy or in thecoating may enter into these compounds. Likewise, when the base metal istantalum or a tantalumbase alloy, the coating constituents will be TaAland Ta Al, respectively.

With any of the above variations of the invention, silicon, chromium,titanium or combinations thereof may also be present in the coating.When silicon is present, it has been found to enter into the ductileeutectic formed at the coating-base metal interface along with thesilver, aluminum, and columbium or tantalum.

Suitable materials which may be alloyed with columbiurn comprisetungsten, molybdenum, tantalum, zirconium, and titanium, this list notbeing a limitation but given only for purposes of illustrating suitablecolumbium-base alloys. It is, of course, desirable that the percentageof alloying ingredient in either columbium or tantalum base alloys notexceed an amount which would preclude the proper formation of aprotective coating on the body.

Columbium alloys containing, about and by weight, tungsten in amounts upto 15 percent, molybdenum from 0 to 5 percent, tantalum from 0 to 33percent, titanium from O to 8 percent, zirconium from 0 to 3 percent,minor amounts of carbon, and the balance essentially all columbium havebeen found to be suitable for high temperature use. A columbium alloywith which the present invention is particularly useful is known as F-48and contains nominally, about and by weight, 15 percent tungsten, 5percent molybdenum, and 1 percent zirconium, balance essentially allcolumbium. Tantalum alloys containing up to 20 weight percent tungsten,balance essentially all tantalum, are also suitable for use with thesecoatings at elevated temperatures.

Space age requirements have created an increasing demand for oxidationresistant structural materials. Some columbium-base alloys such as thetype shown in Patent 2,973,261, Frank, assigned to the same assignee asthe present invention, show excellent strength propert es at 2500 F.However, their use has been limited by their low oxidation resistance.Coating requirements include oxidation resistance, ease of application,and prevention of penetration of oxygen and nitrogen, while at the sametime the coating must cause minimum reduction in ductility afterexposure to air at elevated temperatures. A coating material whichappeared to be attractive was aluminum, forming columbium aluminideswith the proper heat treatment. Of the columbium aluminides, CbAl is themost oxidation resistant but unfortunately is also the most brittle.

It is know that when columbium is given an aluminum base coating in anAg-Al bath the formation of CbAl is suppressed and the only reactionproducts formed are Cb Al and Cb Al. One reason for such action isthought to be that the silver diluting the aluminum causes a decrease inthe aluminum activity. This type of coating results in a more ductilestructure than former aluminide coatings on columbium, but it must beapplied by dipping into a molten metal bath.

According to the present invention, the discovery has been made that acoating of silver prior to the application of an aluminum-base slurrycoating resulted in the formation of a coating predominantly of highlyoxidation resistant CbAl In addition, the silver coating provides at theinterface either a silver rich barrier to crack propagation or a silverrich, ductile grain boundary layer or a combination of the two. Theseeffects persist after oxidation testing for over ten hours at 2500 F. inair. The silver in this form was unexpectedly found to inhibit thepropagation of cracks through the brittle CbAl coating matrix and alsoresulted in an oxidation resistant aluminide coating without causingembrittlement of the base metal. A beneficial effect of the silverplating may be to prevent surface contamination of the base metal byresidual oxygen and nitrogen in the argon atmosphere during the fusiontreatment and before the outer coatings fuse sufficiently to preventsuch contamination.

After a fusion treatment at about 1900 F. for about one hour in anon-oxidizing atmosphere such as argon, a specimen of F-48 columbiumalloy that had been silver plated and then slurry coated with analuminum alloy powder was found to include small entrapped A1 0particles surrounded by a CbAl matrix. X-ray microemission dataindicated that silver and silicon (when an aluminum alloy includingsilicon is used) are uniformly present throughout the coating, probablyin substitutional solid solution be replacement of aluminum atoms in theCbAl phase. Also the silver was somewhat concentrated at the coatinggrain boundaries in the region of the coating-base metal interface.After two hours at 2500" F. in air, the columbium-base alloy coated asabove was observed to have a first inner portion of substantially all CbAl, an intermediate portion of CbAl and an outer portion of A1 0 withsome intermixture of the portions. The coating included progressivelymore columbium toward the columbium alloy-coating interfaw.

In the drawings, FIG. 1 is the microstructure at a magnification of 500xof coated F-48 alloy which has been silver plated and coated with aslurry of Al-l1% Si followed by a slurry of aluminum and then given aone-hour fusion heat treatment at 1900 F. The zones in the coating areidentified in the drawing, a continuous phase of CbAl having dispersedthroughout it particles of A1 0 remaining from the oxygen in the coatingpowders. Silver and silicon are diffused throughout the coating, and theinterface between the coating and the base metal has etched somewhatdifferently than the rest of the coating.

FIG. 2 is a 500 photomicrograph of the coating of FIG. 1 after atwo-hour oxidation treatment at 2500" F. An outer layer of A1 0 and aninner layer of Cb AI have formed and are indicated in the drawing.

FIG. 3 is an electron micrograph at l6,000 of the coated material afterthe fusion treatment of one hour at 1900 F. in a region near theinterface between the coating and the base metal. Particles of the A1 0and a eutectic phase containing silver, silicon and CbAl are shown.

FIG. 4 is equivalent to FIG. 3 except that it is taken on a sample thathas been oxidized in air for two hours at 2500 F. in the region of theinterface between the CbAl and Cb Al zones shown in FIG. 2. The eutecticis better defined than in FIG. 3 and appears at the grain boundaries ofthe CbAl base.

When even minor amounts of this ductile eutectic preferentially form ina substantial portion of the grain boundary area, it acts as a barrierto propagation of cracks from CbAl layer into the base metal. Also, thesilver diffused in solid solution through the CbAl (or its equivalentTaAl may also lessen the bulk brittleness of that phase. In any event,coatings of the present invention including silver do not worsen thenormally poor ductility and high notch sensitivity of high strengthalloys as do similar coatings without the silver content.

The amount of silver required to obtain the advantages of the inventionis structurally defined as that amount which will cause silver to comeout of solution to form a ductile eutectic at a substantial portion ofthe grain boundaries with CbAl or TaAl as columbium or tantalum diffusesinto the coating from the base metal.

In the following examples, percentages are by weight, and mass units arein the avoirdupois system, unless stated otherwise.

The preferred method of applying the coating involves three separatelayers and can be called a silver and double-slurry coating treatment.The specimens coated in the following examples were carefully cleanedand degreased panels of F-48 columbium alloy 1 /2 x /2 x 0.020" of P48columbium alloy with well-rounded edges. First, about 0.001 inch ofsilver is plated on the base metal. This can be done by vapor plating,electroplating, or any other method of applying a coating of silver.This is followed by an aluminum-11% silicon alloy layer and an aluminumovercoat. Both the aluminum alloy and the aluminum are applied in theform of slurries.

One method of silver plating a base metal by means of an electroplatingprocess would begin with careful surface preparation of the base metalby vapor honing and washing in acetone. This is followed by an acid etchfor 20 seconds and subsequent rinsing. The silver plating is done with asilver strike for 45 seconds at 5 volts at room temperature followed bya rinse and then plating in a high speed bath for 5 minutes at 5 voltsat 100 to 120 F. The silver plated piece is then rinsed and dried. Thecompositions used in these steps are as follows:

(A) Acid etch bath- Nitric acid, 20 parts, Hydrofloric acid (48-51%), 20parts, Water, 60 parts.

(B) Silver strike solution- Silver cyanide 0.25 oz./gal., Copper cyanide1.50 oz./gal., Sodium cyanide 10.00 oz./gal.

(C) High speed silver plating bath-- Silver cyanide 14-19 oz./gal.,Potassium cyanide 15-18 oz./gal., Potassium carbonate 2-10 oz./gal.,Potassium hydroxide 4 oz./gal., Ammonium thiosulphate (60%), 2 fluidoz./100 gal.

The silver plated base metal is then ready for the double-slurrycoating. Both of the slurries are prepared by ball milling the mixturesfor a period of 2 to hours. The washed, degreased, and driedsilver-plated panel is dipped into a slurry of aluminum-11% silicon (byweight) prealloyed powder using a dip coater at a speed of 8 r.p.m., andthe piece to be coated is then removed from the slurry and air dried for5 minutes. The same process is then repeated with an aluminum powderslurry with the piece subsequently being dried for 8 hours. A typicalcomposition of aluminum*11% silicon slurry follows:

Grams Al-11% Si powder-325 mesh 100 Xylene 100 Acryloid (an acrylicresin solution) 50 Bentone 34 (dimethyldioctadecyl ammonium bentonite) 2The powder used in these slurries is prealloyed, meaning that ahomogeneous material is formed prior to comminution to powder therebyproducing homogeneously alloyed powder particles. The aluminum slurrymay be composed of:

An Al-1l% Si prealloyed powder is preferred for the first slurry coatbecause of higher fluidity and better wetting of columbium, tantalum andtheir alloys. However, chromium, titanium or silicon can be used invarying amounts up to about 50 weight percent but preferably less thanabout 20 weight percent either as a prealloy or a mixture with thealuminum metal powder, or alternatively, aluminum metal powder can beused without other metals.

The specimens are now ready for heat treatment. They may be suspendedfrom a support beam in an Inconel boat and heat treated inside in anInconel retort in an electric muffle furnace. After the muffle had beeninserted in the retort it is purged with high purity dry argon. Theclosed retort is then put into a preheated furnace. After half an hourat 500 F. when the volatile components in the slurry are driven off, thetemperature is raised to 1900 F. At this temperature all three coatedlayers are liquid. Surface tension will prevent the liquid layers fromrunning off, aluminum oxide covering the surface of the powder particlesin the slurries and increasing the viscosity. The panels are held in anargon atmosphere until the liquid constituents react and solidify toform a coherent, oxidation-resistant coacting. The retort is then pumpeddown to less than one micron vacuum and maintained at 1900 F. foranother half an hour; however, the entire treatment could be carried outin an inert gas atmosphere. Total time of this fusion treatment is aboutone hour at 1900 F.

A preoxidation treatment of exposing a coated and fused article to airat a temperature higher than that of the fusion treatment can be usefulfor conditioning the coating to provide subsequent protection in air atlower temperatures. This effect results in part from forming asubstantial outer layer of A1 0 and in part from some of the innermostC-bAl converting to the more ductile Cb Al with silver and perhapssilicon precipitating from solution in the inner layers. Thisprecipitated silver-silicon phase eventually migrates to the grainboundaries and there causes further ductilization. The preoxidationtreatment is optional and would be of benefit primarily when the coatedarticles is to be be exposed to an oxidizing atmosphere at relativelylow temperatures such as below 2300 F. Articles to be exposed to air athigher temperatures will undergo this conditioning in use and a separatepreoxidation treatment would be of less value for such articles.

It will readily be seen by those skilled in the art that thepreoxidation conditioning treatment will be beneficial even if only oneof the two preferred conditions is met. An oxidizing treatment withoutthe higher temperature will produce the A1 0 outer layer. A highertemperature than the fusion treatment temperature in a nonoxidizin-gatmosphere will cause the formation of the Cb Al inner layer. However,maximum benefit is or tained from a preoxidation treatment by using ahigher temperature than the fusion temperature such as from 2000 F. to2500 F., preferably about 2300 F., and doing the heating in an oxidizingatmosphere such as air.

Standard oxidation tests of the coated panels were carried out in anelectric muffle furnace in static air at 2500 F. for two hours. Everyhalf hour the samples were taken out of the furnace and inspected forfailure. After a ten-hour exposure, the coating showed no sign of edgefailure and the bend ductility of the parent or base metal was excellentat room temperature.

Bend tests were performed in a 90 fixture with a 2T radius mandrel onsilver-plated and double-slurry coated F-48 columbium alloy panelscoated as described above. These tests showed a 90 bend angle at roomtemperature both after the fusion treatment of one hour at 2500 F. inargon and after subsequent oxidation of two hours at 2500 F. in air.

The coating-base metal system and methods for applying the coatingsdescribed in this specification and in the appended claims represent asignificant improvement in the art of aluminum-base coatings forcolumbium and tantalum and alloy systems based on columbium or tantalum.A considerable improvement is found in the ductility properties ofbodies coated in accordance with this invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. An article of manufacture comprising a body of a base metal selectedfrom the group consisting of columbium, columbium-base alloys, tantalum,and tantalumbase alloys, and a protective coating integral with anexposed surface of said body to protect it against oxidation while notdestroying the ductility of the base metal by greatly increasingsensitivity to crack propagation, said coating being bonded together andto said base metal by diffusion and comprising:

a continuous layer of a continuous phase of a monometal trialuminidehaving a composition in the proportion of one atom at leastpredominantly of the selected one of the base metals columbium andtantalum to each three atoms of aluminum,

said layer containing sufficient amounts of silver in solution so thatan excess of the selected one of the base metals columbium and tantalumdiffuses into said layer upon exposure to elevated temperatures and anoxidizing atmosphere, a ductile eutectic containing silver, aluminum andthe selected one of the base metals columbium and tantalum forms at asubstantial portion of the grain boundaries of said monometaltrialuminide and acts as a barrier to the propagation of cracks fromsaid layer into said base metal.

2. An article of manufacture according to claim 1 which, upon continuedexposure to elevated temperatures and an oxidizing atmosphere, forms acontinuous layer of A1 at its exposed surface and a continuous layer ofa di-metal aluminide having a composition in the proportion of two atomsat least predominantly of the selected one of the base metals columbiumand tantalum 5 to each atom of aluminum between said monometaltrialuminide and said base metal.

3. An article of manufacture according to claim 1 in which the coatingalso contains at least one other metal selected from the groupconsisting of silicon, chromium and titanium.

4. An article of manufacture according to claim 1 in which said ductileeutectic contains silicon in addition to silver, aluminum and theselected one of the base metals columbium and tantalum.

5. An article of manufacture according to claim 2 in which the basemetal is a columbium-base alloy containing, about and by weight,tungsten in amounts up to 15 percent, molybdenum from 0 to 5 percent,tantalum from 0 to 33 percent, titanium from 0 to 8 percent, zirconiumfrom 0 to 3 percent, minor amounts of carbon, and the balanceessentially all columbium, and

in which the monometal trialuminide is predominantly CbAl and thedi-metal aluminide is predominantly Cb Al.

6. An article of manufacture according to claim 5 in which the basemetal is a columbium-base alloy consisting essentially of, about and byweight, 15 percent tungsten, 5 percent molybdenum and 1 percentzirconium, balance columbium.

7. An article of manufacture according to claim 2 in which the basemetal is a tantalum-base alloy containing tungsten in amounts up toabout 20 weight percent and the balance essentially all tantalum, and

in which the monometal trialuminide is predominantly TaAl and thedi-rnetal aluminide is predominantly TH2A1- References Cited by theExaminer UNITED STATES PATENTS 3,186,070 6/1935 Oxx 29198 X 3,205,0909/1965 Caplow 29l97 X 3,206,289 9/1965 Bradley 29l97 HYLAND BIZOT,Primary Examiner.

1. AN ARTICLE OF MANUFACTURE COMPRISING A BODY OF A BASE METAL SELECTEDFROM THE GROUP CONSISTING OF COLUMBIUM, COLUMBIUM-BASE ALLOYS, TANTALUM,AND TANTALUMBASE ALLOYS, AND A PROTECTIVE COATING INTEGRAL WITH ANEXPOSED SURFACE OF SAID BODY TO PROTECT IT AGAINST OXIDATION WHILE NOTDESTROYING THE DUCTILITY OF THE BASE METAL BY GREATLY INCREASINGSENSITIVITY TO CRACK PROPAGATION, SAID COATING BEING BONDED TOGETHER ANDTO SAID BASE METAL BY DIFFUSION AND COMPRISING: A CONTINUOUS LAYER OF ACONTINUOUS PHASE OF A MONOMETAL TRIALUMINIDE HAVING A COMPOSITION IN THEPROPORTION OF ONE ATOM AT LEAST PREDOMINANTLY OF THE SELECTED ONE OF THEBASE METALS COLUMBIUM AND TANTALUM TO EACH THREE ATOMS OF ALUMINUM, SAIDLAYER CONTAINING SUFFICIENT AMOUNTS OF SILVER IN SOLUTION SO THAT ANEXCESS OF THE SELECTED ONE OF THE BASE METALS COLUMBIUM AND TANTALUMDIFFUSES INTO SAID LAYER UPON EXPOSURE TO ELEVATED TEMPERATURES AND ANOXIDIZING ATMOSPHERE, A DUCTILE EUTECTIC CONTAINING SILVER, ALUMINUM ANDTHE SELECTED ONE OF THE BASE METALS COLUMBIUM AND TANTALUM FORMS AT ASUBSTANTIAL PORTION OF THE GRAIN BOUNDARIES OF SAID MONOMETALTRALUMINIDE AND ACTS AS A BARRIER TO THE PROPAGATION OF CRACKS FROM SAIDLAYER INTO SAID BASE METAL.